1. Field of the Invention
This invention relates to circuit breakers and, more particularly, to circuit breakers with mechanism that trip the circuit breaker when voltage in the protected circuit drops below a specified value.
2. Background Information
Circuit breakers primarily provide protection from excessive current in an electric power distribution system. Some circuit breakers additionally provide protection against low voltage that could damage equipment in the protected circuit. Typically, low voltage protection is provided by an undervoltage release (UVR) that includes a coil energized by the protected circuit. As long as the voltage remains above a dropout level, the magnetic force generated by the coil is sufficient to maintain a plunger in a retracted position. However, when the voltage drops below the specified value, a compression spring overcomes the magnetic force and extends the plunger out of the coil to actuate the trip mechanism of the circuit breaker. The magnetic force generated by the coil is not sufficient to retract the plunger against the bias of the compression spring even if it returns to full value. Consequently, a reset lever is provided to mechanically retract the plunger. It is known to actuate the reset lever when the handle of the circuit breaker is returned to the off position. A pin on the handle arm directly contacts the reset lever on the overvoltage relay as the handle reaches the off position. However, the handle needs to be moved beyond the off position to reset the circuit breaker operating mechanism. This results in overtravel of the pin on the handle arm.
In the past, this problem has been addressed by relying on the undervoltage relay to absorb any overtravel. Unfortunately, with variations in tolerances, excessive stress may occur on the undervoltage relay, and its functionality may be compromised.
There is room, therefore, for improvement in undervoltage release mechanisms for circuit breakers. There is also room for improvement in circuit breakers employing undervoltage release mechanisms.